linux-kernel - Re: [PATCH 08/11] vfs: Merge check_submounts_and_drop and d

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Date:	Tue, 25 Feb 2014 14:03:36 -0800
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Miklos Szeredi <miklos@...redi.hu>,
	Al Viro <viro@...iv.linux.org.uk>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Rob Landley <rob@...dley.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christoph Hellwig <hch@...radead.org>,
	Karel Zak <kzak@...hat.com>
Subject: Re: [PATCH 08/11] vfs: Merge check_submounts_and_drop and d_invalidate

"J. Bruce Fields" <bfields@...ldses.org> writes:

> On Mon, Feb 24, 2014 at 04:01:29PM -0800, Eric W. Biederman wrote:
>> Miklos Szeredi <miklos@...redi.hu> writes:
>> 
>> >
>> > You can optimize this by including the negative check within the above d_locked
>> > region and calling __d_drop() instead.
>> 
>> For this patch just moving the code and not changing it is the corret
>> thing to do because it helps with review and understanding the code.
>> 
>> There are two ways I could see going with optimizing the preamble.
>> Simply dropping the d_lock from around the d_unhashed test as a pointer
>> dereference should be atomic, and the test is racy against
>> d_materialise_unique.
>
> Could you explain?  What's the race, and what are the consequences?

>> (We don't always hold the parent directories inode mutex when d_invalidate is called).

d_unhashed is not a permanent condition because of d_materialise_unique,
and d_splice_alias.

d_invalidate can be called on an unhashed dentry in one of two ways
(either d_revalidate dropped the dentry or another routine that drops
the dentry beat the current invocation of d_invalidate to the job).

There are 3 places d_revalidate is called.

Once on the rcu path with with the appropriate flag set.

Once without out the parent i_mutex held, just off of the rcu path,
on that path d_invalidate is when d_revalidate fails.

Once during lookup with the parent directory i_mutex held.

Because the parent direcories i_mutex is not always held accross
d_revalidate and the following d_invalidate it happens that d_invalidate
is not always an atomic operation.

At worst the race results in a dentry that is dropped when it could be
hashed, that we will resurrect next time someone attempts to look it
up and d_materialise_unique/d_splice_alias is called.

None of that really matters for optimizing d_invalidate, but it is part
of the background in which d_invalidate lives. All that is significant
in d_invalidate is knowing that d_materialise_unique, and possibly
d_splice_alias may run concurrently with d_invalidate.  It is unlikely
and essentially harmless.

After my patchset (because I removed all of the d_drop's from
.d_revalidate) the only race that should remain is between two parallel
calls of d_invalidate.  Which probably means we can remove the test for
d_unhashed altogether.

Right now I just want to make this first big step and make certain the
code is solid.  After that optimization is easy.

Eric
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